Any word on how the Aug 22nd nerf to Critical Mass affects Fire overall? Will this work as a buff while leveling / Dungeon Geared and nerf with Raid Gear? Any clues?
If your char sheet crit chance post change is 32.5% or less it's a straight buff to all spells against an equal level target. Against a dungeon boss (+2 lvls from you) 34.5% or less is a buff, against a raid boss (+3 lvls) 35.5% is a buff.
Why do you list 3 numbers? 32.5, 34.5, 35.5?
Your stat sheet critical strike chance is suppressed 1% for each level that your target is above you. Heroic/Mythic dungeon bosses are always 2 levels above you, and raid bosses are always 3 levels above you. Hence, the breakeven points (that is, the critical strike chance at which the 5% baseline buff to crit offsets exactly the nerf to the critical mass multiplier) will vary depending on the level of your target. The most useful numbers to know are for fighting same-level mobs, for fighting dungeon bosses, and for fighting raid bosses; hence the numbers she or he provided.
To find the breakeven point for same target mobs:
Let x represent your pre-change character sheet crit chance
1.3x = 1.1(x+5)
x = 27.5
To find the breakeven point against raid boss mobs:
Let x represent your pre-change character sheet crit chance
1.3(x–3) = 1.1(x+5–3)
x = 30.5
The post-change stat sheet crit will simply be 5% added to the value of x.